As is well known and understood, perhaps the most common method of reducing continuous wave interference is through the use of band pass notch filters. However, as applied in audio communications systems-- and, especially, in audio avionics defense communication-- the use of such filters are disadvantageous in certain respects. First of all, relatively large, and therefore expensive, inductors and capacitors are required for the passive, low frequency filtering. Secondly, the filtering also distorts the voice signal and deteriorates its intelligibility. Thirdly, the filtering is done around a pre-set frequency, affording the possibility of signal jamming merely by changing frequencies.